RUST - The Enemy of All Car Owners Part 1 - Why Iron Rusts

Rust is the enemy of all car owners. Rust is that nasty brown stuff which destroys the bodywork of our cars, which causes our paintwork to bubble and blister, which is the downfall of our chassis and which causes our car to fail that vital MOT. Rust is the final dust into which our cars will crumble.

Rust is the oxide of iron that results from a chemical reaction: the oxidation of iron by atmospheric oxygen. There are in fact three kinds of iron oxides. These are known chemically as FeO, which is a black powder, Fe2O3 which is a brown-red powder, and Fe3O4 which is a mixture of the other two.

Iron will only rust in the presence of moisture. If you had an iron nail in a jar of oxygen with absolutely no moisture present the iron nail would never rust. Add some moisture and rusting will happen quite quickly, add a sprinkle of salt and it will happen very rapidly indeed.

When things go rusty a quite complex electro-chemical process is taking place. When iron is in contact with water that contains some dissolved oxygen, something called a solution tension is created. This will vary over the surface and will result in different electrical potentials. In the relatively electropositive regions the iron will dissolve forming positive iron II ions. At the electronegative regions negative hydroxyl ions (OH-) are formed. These react with the iron to form iron hydroxides which are insoluble. The iron II ions that are in solution react with hydrogen ions to form iron III ions which react with the hydroxide ions to form hydrated iron oxides. When these dry out they form rust. The greater the initial number of hydrogen ions (the lower the pH) and the more conductive the water (for instance the greater the concentration of road salt) the faster the rusting takes place.

When the rusting process has gone too far, the car parts that have corroded are irretrievable and will need to be replaced. Nowadays it is relatively easy to purchase these online, however prevention is better than cure and in the next article of this series we will examine ways to prevent rust.